The present invention relates to a tunable filter using a diffraction grating for selectively passing only light of an arbitrary wavelength.
As a tunable filter for selectively passing only light of an arbitrary wavelength, there is the tunable filter using a band-pass filter.
Further, as a tunable filter using a diffraction grating, it is applied from the same applicant as the Japanese Patent Application No. 2001-066682.
FIG. 8 is a diagram showing one example of a tunable filter described in the application using a diffraction grating which is a premise of the present invention as a reference example.
In FIG. 8, numeral 13 is a two-core fiber for both input light and output light, and numeral 5 is a condenser lens.
Incidentally, the condenser lens 5 may be a GRIN lens (gradient index lens).
Further, numeral 9 is a diffraction grating, and numeral 10 is a parallelogram prism type polarizer, and numeral 7 is a polarization rotation element. The polarization rotation element includes a half-wave plate or a garnet thick film, and as the wave plate, a zero-order wave plate capable of use at a wide wavelength band is desirable.
Further, numeral 12 is a total reflection mirror and is constructed rotatably as shown by an arrow of the drawing in order to change an angle again launched into the diffraction grating after reflecting light from the diffraction grating 9.
By this configuration, a selection of a wavelength outputted can be made by rotating the total reflection mirror 12 as shown in the drawing to adjust an angle with respect to input light of the diffraction grating.
Then, while the plane of polarization with low diffraction efficiency of the diffraction grating of one of the polarized waves split by the parallelogram prism type polarizer 10 is polarized and rotated 90° by the polarization rotation element 7 and is launched to the diffraction grating 9, the plane of polarization of the other split by the parallelogram prism type polarizer 10 is reflected by a reflection surface of the parallelogram prism type polarizer 10 and is launched to the diffraction grating 9 and both the planes are joined by the condenser lens 5.
In this example, by the total reflection mirror 12, light from the diffraction grating 9 is reflected and is again returned to the diffraction grating, so that the light passes through the diffraction grating two times and selectivity of a wavelength can be improved more.
Further, by using the two-core fiber 13, it has a feature that one condenser lens will suffice and an apparatus can be constructed at low cost.
Since the filter described in FIG. 8 shown as the reference example described above does input and output of light opposed to the condenser lens by the two-core fiber, cores of fibers forming the two-core fiber are arranged with the cores offset by a radius (for example, 62.5 μm) of the fiber from the center of the lens, so that the following problem arises.
The problem will be described in detail using FIGS. 9 and 10.
FIG. 9 is a diagram showing an arrangement relation among a two-core fiber 2-1, a condenser lens 2-2 and a total reflection mirror 2-3.
In FIG. 9, an input fiber 2-1a and an output fiber 2-1b forming the two-core fiber 2-1 are arranged with the fibers respectively offset (shifted) by a radius (for example, 62.5 μm) of the fiber from the center 2-4 of the lens 2-2 as shown in the drawing.
In order to couple light emitted from the input fiber 2-1a to the output fiber efficiently in such arrangement, it is necessary to arrange the total reflection mirror 2-3 in a length (F′) equal to a focal length (F) of the condenser lens 2-2 (that is, coupling efficiency becomes maximum in the case of F=F′).
However, in the tunable filter, a polarization split element, a ½λ wave plate and a diffraction grating are arranged between a condenser lens and a total reflection mirror as shown in FIG. 8 and a long optical path length is required, so that there is a problem that arrangement balance is lost and coupling efficiency becomes worse as shown in FIG. 10.
In FIG. 10, in a manner similar to FIG. 9, the input fiber 2-1a and the output fiber 2-1b forming the two-core fiber 2-1 are arranged with the fibers respectively offset (shifted) by a radius (for example, 62.5 μm) of the fiber from the center 2-4 of the lens 2-2 as shown in the drawing.
Then, the polarization split element, the ½λ wave plate and the diffraction grating are arranged between the condenser lens and the total reflection mirror, a length of F′ becomes long (that is, the coupling efficiency becomes worse in the case of F<F′).
Therefore, in order to improve deterioration of the coupling efficiency in the case of FIG. 10, a condenser lens with a long focal length (F) is used, but in the lens with the long focal length (F), not only its outer diameter becomes large but also a beam diameter becomes thick.
Because of that, the used polarization split element, the ½λ wave plate, the diffraction grating and the total reflection mirror with the size corresponding to the lens become necessary and an optical path length also becomes long, so that a problem that the whole apparatus becomes large arises.